Two accounts with separate roles cannot be merged together. A Constituent cannot be merged with a Tribute account, a Tribute cannot be merged with a User, so on and so forth.
If you are unable to find the second account when you're attempting to set up the merge, this account likely has a different role than the one you currently have open. If you have a Constituent account open and you click Merge Role to search for the second account, eTapestry will only show you Constituent accounts. If the second account for that individual is a Tribute account, you will not be able to find that account in your search.
You can verify this by searching for the second account and opening it, then looking in the upper right hand corner where the account number displays. eTapestry will display in this same section the role assigned for that account: